The invention relates to a flat cable extensively used in various types of electric and electronic equipment, such as audio/visual (AV) equipment and office automation (OA) equipment, and more particularly to a flexible flat cable (FFC) having excellent flex resistance and heat resistance.
Flat cables (FCs), which have hitherto been used as circuit-to-circuit jumper wires (fixation wires) or the like in various types of electric and electronic equipment, are multi-core strip-type electric wires comprising several to ten-odd conductors which have been integrally covered with an insulating cover. By virtue of particularly excellent flexibility (flex resistance), in recent years, the flat cables have become more and more applied as an alternative to flexible printed wiring boards (FPC) which have been used as a wiring material in movable parts of AV equipment, OA equipment and the like.
For example, in 0.1 to 0.3 mm-thick flexible flat cables (FFCs) using a pure tin or solder plated flat-type conductor (a pure copper system (for example, TPC)), the use of the FFCs, for example, in photo-pickup wiring of CD players for music and CD-ROM of personal computers and car navigation systems has been rapidly increased from the viewpoint of reducing prices. Further, in recent years, the development and mass production of high-density FFCs of 22 xcexcm in conductor thicknessxc3x970.5 mm in pitch have been realized, and these FFCs have become applied to the latest electronic equipment, for example, photo-pickup wiring of DVDs (digital versatile discs).
A tendency of CDs and DVDs toward higher performance and higher compactness has lead to a fear that the internal temperature reaches 60xc2x0 C. and, in some products, up to 80xc2x0 C. To cope with this, excellent flex resistance at high temperatures as well as room temperature is required of flexible flat cables used in these applications.
In most of the conventional flexible flat cables, PET (polyethylene terephthalate) is used as an insulating cover (a plastic film), and polyester is used as an adhesive. Due to this constitution, the flex resistance at 60xc2x0 C. is lowered to about 20% or less of the flex resistance at room temperature. Therefore, at the present time, the conventional flexible flat cables are not used in all CDs, DVDs and the like.
The invention has been made with a view to solving the above problems of the prior art, and it is an object of the invention to provide a novel flexible flat cable having a combination of excellent flex resistance and heat resistance, and a process for producing the same.
According to the first feature of the invention, a heat-resistant, flex-resistant flexible flat cable comprises: a single tin or solder plated conductor or a plurality of tin or solder plated conductors juxtaposed to each other or one another; and a pair of plastic films with an adhesive, said single conductor or said plurality of conductors having been sandwiched between the pair of plastic films with an adhesive to form an integral structure, said plastic films with an adhesive being formed of a polyimide with an adhesive which has a modulus of longitudinal elasticity of not less than 300 kg/mm2 and an elongation of not less than 20%, said adhesive being formed of an epoxy having a glass transition temperature Tg of 80xc2x0 C. or above, the 180xc2x0 peel strength between the adhesive and the conductor being not less than 0.5 kg/cm.
According to this constitution, the flat cable has significantly improved heat resistance, and can exhibit excellent flex resistance even at a high temperature of 80xc2x0 C. or above.
In this flat cable, the epoxy constituting the adhesive is preferably cured at a temperature of 100xc2x0 C. or below. This can prevent a lowering in strength of the conductor, which adversely affects the flexibility, and the diffusion of plating ingredients into the epoxy during the step of curing.
According to the second feature of the invention, a process for producing the heat-resistant, flex-resistant flexible flat cable according to the first feature of the invention, comprises the steps of:
sandwiching and temporarily applying a single conductor or a plurality of conductors between the pair of plastic films with an adhesive having the above properties by means of a hot roll; and
then heating the laminate in a heating oven at a temperature of 100xc2x0 C. or below for several hours to several tens of hours to cure the adhesive.
According to this constitution, the flexible flat cable having excellent heat resistance and flex resistance can be easily produced.